I thought I was going mad. My Squadra Hometime had been accurate to around 2-4 seconds per day which was fine. Then one day, I got home and it had gained 24 seconds in the space of 12 hours.
I took it to the AD to be checked and calibrated, and picked it back up today. The cause of the problem was that the watch was magnetized by the arch I walked through at the airport on the day it gained 24 seconds. Once it was demagnetized it went back down to +4 seconds, and it has now been calibrated further and has gained 1 second in the past 24 hours.
So, from now on I will only be passing my watches through the lower intensity scanner in a plastic tray. One odd thing is that my EWC has been through the arch a couple of times and is still running at a net accuracy of 0 seconds (not gaining or losing time at all!). I wonder if the titanium case makes a difference to minimise magnetizing, or whether the exposing of the HT movement in the sapphire glass made it more prone to the magnetic field.
Simon